Owners, operators and maintainers of DHC-3 aeroplanes
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The purpose of this Civil Aviation Safety Alert (CASA) is to raise awareness of the importance of the proper installation of the stabilizer actuator lock ring on the DHC-3 aeroplane.
A fatal DHC-3 aeroplane accident occurred on 4 September 2022 at Mutiny Bay, near Freeland, Washington, U.S.A. Witnesses reported that the aircraft was in level flight before it entered a slight climb, then pitched down in a near-vertical descent. The aircraft continued in a nose-low, near-vertical descent until it impacted water.
The National Transportation Safety Board (NTSB) is carrying out the accident investigation and has recently issued an investigative update. The NTSB noted in the update that the stabilizer actuator clamp nut on the accident aeroplane separated from the stabilizer barrel by unthreading and the lock ring securing the clamp nut to the barrel was missing.
Since the upper end of the actuator acts as the rear attachment for the stabilizer, separation of the clamp nut from the actuator barrel will result in the stabilizer being attached only by the two forward pivot points, resulting in a free-floating stabilizer with the potential of reduction or loss of pitch control.
The NTSB does not know whether the lock ring was installed on the accident aeroplane, but it noted that it is possible for the lock ring to be partially installed if its tang is not fully inserted into the actuator barrel and clamp nut hole.
Based on the current accident findings, Viking Air Limited issued a Service Letter DHC3-SL-27-001 to recommend a one-time inspection of the actuator to:
- confirm presence of actuator lock ring
- confirm lock ring is seated in the groove in the upper housing
- confirm lock ring tang is engaged in the clamp nut
Transport Canada continues to review all available data to determine if any other corrective action should be implemented.
Transport Canada recommends that owners, operators and maintainers carry out the inspection and corrective actions described in Viking Air Limited Service Letter DHC3-SL-27-001 and report the findings to Viking Air Limited using the Stabilizer Actuator – Lock Ring Inspection Report attached to the Service Letter.
In reviewing the NTSB investigative update, Transport Canada noticed that an additional red-coloured seal was installed between the actuator clamp nut and upper bearings. Review showed that the seal is not part of the aeroplane type design. Therefore, Transport Canada also recommends that when the actuator is disassembled to carry out the periodic lubrication and inspection requirements, owners, operators and maintainers should ensure that the configuration of the actuator components is according to the type design.
For more information concerning this issue, contact a Transport Canada Centre; or contact Philip Tang, Continuing Airworthiness in Ottawa, by telephone at 1-888-663-3639, by fax at 613-996-9178 or by e-mail at TC.CAWWEBFEEDBACK-retroactionWebMDLN.TC@TC.GC.CA.
Original signed by
Acting Chief, Continuing Airworthiness
National Aircraft Certification
The Transport Canada Civil Aviation Safety Alert (CASA) is used to convey important safety information and contains recommended action items. The CASA strives to assist the aviation industry's efforts to provide a service with the highest possible degree of safety. The information contained herein is often critical and must be conveyed to the appropriate office in a timely manner. The CASA may be changed or amended should new information become available.